Mutations in myotubularin cause X-linked myotubular myopathy (XLMTM), a devastating congenital muscle disorder causing severe muscle weakness and premature death from respiratory muscle failure. Only supportive, palliative care is available. We have shown that AAV-mediated gene replacement rescued severe muscle weakness in myotubularin-deficient mice. New data show that a single systemic treatment with rAAV-MTM1 sufficed for long-term (at least one year) survival and essentially complete amelioration of symptoms of mice with myotubularin-deficient muscles. However, for eventual therapy of XLMTM patients, it is imperative to employ a predictive large animal model to refine the delivery system, assess critical safety parameters such as the potential host immune response to vector and transgene, and optimize efficacy measurements. We, therefore, developed a breeding colony in which affected male dogs display a phenotype directly analogous to human XLMTM. In preliminary studies we confirmed that local gene replacement therapy, delivered intramuscularly in the hind limb of young dogs, improved both function and pathology in myotubularin-deficient skeletal muscles. Moreover, the first XLMTM dog in which rAAV-MTM1 was delivered systemically maintained nearly normal strength in all four limbs, and normal respiratory function for more than 6 months surviving > 4 months longer than any untreated mutant dog in our colony. The positive observations in the murine and canine models drive us to focus on the development of systemic rAAV-MTM1 gene therapy. We hypothesize that modest levels of myotubularin will suffice to sustain long-term functionality of striated muscles throughout the body, including the vital respiratory muscles. We propose to utilize the canine system to test this hypothesis and to optimize vector selection (Aim 1) and dosing (Aim 2), while assessing potential safety concerns. Towards this end our Specific Aims are: Aim 1. Test systemic MTM1 gene replacement in the canine model using a novel rAAV vector (serotype 2i8) engineered for effective delivery to skeletal muscle while avoiding the liver Aim 2. Determine dose-response relationships for safety, efficacy, and immune response parameters over a period of at least 32 weeks after systemic MTM1 gene replacement in young XLMTM dogs